Closed combustion cycle for cement kilns



Sept. 9, 1969 A LA VELl-E 3,466,351

CLOSED COMBUSTION CYCLE FOR CEMENT KILNS Original Filed May 23, 1966 2Sheets-Sheet 1 :E'I [3"..1A

mvzsmoa Martin J. LoVelle ATTORNEYS Sept. 9, 1969 M. J. LA VELLE CLOSEDCOMBUSTION CYCLE FOR CEMENT KILNS 2 Sheets-Sheet 2 Original Filed May23. 1966 INVENTOR.

United States Patent US. Cl. 26353 4 Claims ABSTRACT OF THE DISCLOSUREIn heating devices, such as rotary cement kilns, performance can beimproved and the spewing, of combustion products and dust into theatmosphere can be eliminated by a completely closed combustion processwherein a synthetic gas mixture is continuously circulated within thedevice with fuel and oxygen added to the circulating synthetic gaseousmixture under combustion conditions to add thermal energy for heatingwhile removing 1 a quantity of the circulating synthetic gaseous mixtureto provide for the additional gas volume generated by the combustion ofthe fuel and oxygen. The removed quantities of said mixtures can beprocessed to remove undesirable air contaminants or largely recovered asby-products.

This application is a continuation of Ser. No. 551,995, filed May 23,1966, which is now abandoned.

This invention relates to the combustion of fuels, and more particularlyto an improved combustion process and devices wherein the fuel is burnedwhile mixed with a synthetic gas mixture composed primarily of CO H O(vapor) and 0 instead of the conventional mixture of fuel and air.

This process technique of this invention can be employed in sustainedflame systems, such as boilers, or intermittent cyclical flame systemswith many advantages over conventional processes.

Conventional combustion processes usually involve the burning of fuelwith air in a combustion zone and a subsequent utilization of the heatenergy released to ac complish some useful work function. Air is used assource of oxygen which is necessary for combustion since it is readilyavailable. Air, however, contains only about 21% by volume of oxygen,with the balance composed chiefly of nitrogen which is inert in thecombustion process. Thus to have a workable combustion process, it hasbeen necessary to circulate a gas volume approximately five times thatof the oxygen necessary if it were pure form, instead of contained inthe air.

Such a large gas mass, of it inert in the combustion process, lowers theefficiency, since the temperature of the inert gases is raisedconsiderably within the system and then discharged to the atmosphere,with a resultant heat loss. Further, the large gas mass flow requireshigher velocities within the system, lowering heat transfer efficiencytherefrom to some useful work function.

The large gas volume flow in conventional combustion processes may beillustrated by referring to combustion engineering literature, whichshows that 214 cubic feet of air at standard conditions are necessaryfor the combustion of a pound of methane. Further, dependent on thespecific characteristics of the particular system, up to 100% excess airmay be required, or 428 cubic feet of air per pound of methane. Ifoxygen were used in place of air, only 43 cubic feet would be required.

Thus, in conventional combustion processes at least 20% of the thermalenergy liberated is retained in the ice inert nitrogen and is lost tothe atmosphere through the stack gases along with the products ofcombustion, CO H O. The magnitude of this heat loss to the atmospherethrough stack gases represents one of the chief disadvantages of theconventional combustion processes.

Still another disadvantage of the conventional combustion process isincomplete conversion of carbon and hydrogen components of fuels totheir respective oxides and the liberation of the maximum quantity ofthermal energy which is virtually impossible to achieve and maintainwith any degree of consistency. Incompletely burned combustible materialinvariably escapes to the atmosphere as partially oxidized substances orto the ash residue as unburned portions of the fuel. The magnitude ofthis loss is a direct function of the load on the combustion system andthe loss of combustible material increases with increased energy demandand increased fuel utilization.

The significance of the discharge of incompletely combusted fuelproducts to the atmosphere has become of greater concern in recent yearsdue to the increased awareness of the public to the problems of airpollution. Because of air pollutants in stack emission of conventionalcombustion processes, industrial management has been required to takesteps to prevent air pollution due to these materials emanating fromtheir stacks. While control devices such as mechanical and electrostaticdust collectors have been successful in removing particulate matter fromstack gases, within the design ranges of the equipment, the cost ofhandling the material collected has resulted in serious operatingproblems for plant management. In addition, the conventional devices areineffective against the pure carbon, smog producing hydrocarbons andmonoxides, which are serious forms of air pollution in urbancommunities.

In addition to the foregoing, the reliance on atmospheric air as asource of oxygen for combination of hydrocarbon fuels has prevented anysignificant progress toward development of a more compact combustionsystem. All combustion processes must be designed for a wide latitude ofload conditions if they are to be flexible. Changes in load conditionsmanifest themselves as significant changes in gas flow within a furnacedevice. For example, an increment of pounds of methane per hour over aprevious fuel feed setting in a conventional combustion system theremust be a provision in the device for the increased gas volume toaccommodate from 21,400 to 42,800 cubic feet of additional air flow toburn the additional fuel. This example represents a very minor change inload ranges for any typical industrial application of the combustionprocess, but illustrates the problem.

Conventional combustion systems also have another disadvantage which canbe attributed to their dependence on atmospheric air which causes largeamounts of nitrogen to be introduced into the system. This is theinability to economically recover the -by-products of combustion, namelyH 0 and CO which has not previously been feasible due to the dilution ofthese combustion products by the large volume of inert nitrogen passingthrough the system. For example, the average stack concentration of theH 0 and CO components is generally only 6 and 10% of the eflluent of arotary kiln stack, the remainder being principally nitrogen and excessoxygen.

It is therefore apparent that the thermal and material inefficienciespresent in the conventional combustion system represent disadvantages ofthe present method of liberating heat energy in hydrocarbon fuelsthrough combustion processes. It is therefore evident that there is aneed for new combustion processes which avoid the deficiencies inconventional systems.

Accordingly, it is the object of this invention to provide a new, moreefficient combustion process for the generation of thermal energy and asubsequent, more efiicient application of that energy to a useful workfunction.

Another object of the present invention is a novel combustion processwhich substantially reduces air pollution resulting from the combustionof hydrocarbon fuels while significantly increasing the degree ofmaterial and thermal efiiciencies of the process.

Another object of the invention is to provide an mproved combustionmethod which can be incorporated at modest cost to any existingcombustion system and will subsequently improve the efficiency of heatgeneration and its utilization which have been previously established inthe particular combustion system so modified.

An important object of the present invention is a novel combustionprocess which can be varied between its maximum and minimum heatgenerating ranges with no appreciable change of material or thermalefficiencies.

Still another object is the provision of a novel combustion processwhich can be contained in much smaller space than can conventionalprocesses, with the same heat generating capacity.

An ancillary object of the invention is to provide a new combustionprocess where the valuable by-products of combustion, namely CO and H 0,are sufficiently concentrated to make their recovery practical.

An important object of the invention is the provision of a circulatinggaseous mass in the combustion system, which has a greater specific heatcapacity and better diathermanous properties than the gas massesemployed in conventional combustion pocesses.

A more specific object of the invention is the provision of a combustionprocess wherein the loss of thermal energy through stack gases is almosteliminated.

Another more specific object of the invention is the provision of anovel process where poor grades of fuels such as those having a highsulfur content, can be utilized in place of the premium grade fuels,with no draw-backs.

The foregoing objects, as well as many others, can be accomplished in anovel combustion system wherein the fuel is burned with a synthetic gasmixture composed primarily of carbon dioxide, water vapor and oxygeninstead of air as is used in conventional combustion systems. Generallythe process is carried out by recirculating a substantial portion of thehot gaseous products of combustion through combustion Zones and mixing asufficient amount of oxygen with these gases to support the combustionof the fuel. From 1% to 50% by volume of oxygen can be added to hotgaseous products of combustion to form a hot synthetic gas mixturecapable of supporting the combustion of fuels. More specifically, thecomplete novel combustion system in this invention also includes therecovery of excess by-products of combustion, principally carbon dioxideand water.

In the drawings FIGS. 1A and 1B diagrammatically illustrate a rotarykiln for making cement which has been modified for the practice of thisinvention. Since the invention has particular applicability to suchrotary kilns it will be described in this environment, but it is notintended that it be limited thereto.

Referring to the drawings, a refractory-lined kiln is shown, which has arotating cylindrical shell 11 supported on concrete piers 12 at a slightincline with rollers 13 through shell rims 14. Rotation of the shell isaccom plished by a shell-mounted girth gear 15 driven by pinion 16powered via gear reducer 17 by motor 18 with the latter mounted on acentrally located pier 19.

The ends of the rotating shell 11 are closed by hoods and connectedthereto by sealing ring members 20. A refractory-lined firing hood 21closes the lower end of the shell, and a dust collecting hood 22 closesthe upper end. The sealing ring members prevent gas leakage into or outof the stationary hood structures as the shell is rotated about itslongitudinal axis by the motor.

Raw feed to the kiln is introduced through a waterjacketed, slurry feedpipe 23 which extends through the dust collecting hood 22 into the upperend of the rotating shell 11. The slurry feed pipe is fed from aferris-wheel feeder 24 which picks up feed from the slurry supply line25, dumping it into a receiving tank 26. The ferris-wheel feeder isdriven by a belt or chain 27 connecting it with a driving structure onthe rotating shell 11.

At the opposite end of the kiln from the dust collecting hood 22 whichis closed by firing hood 21, a burner pipe 29 is mounted in the firinghood so that it terminates in the lower end of the rotating shell 11.This burner pipe discharges .a combustible mixture of fuel from fuelhopper 30, and an oxygen containing gas from primary blower fan 31 intothe rotating shell where it is burned to provide direct heat in thekiln.

The bottom of the firing hood 21 is open so that the material dischargedinto the firing hood from the shell 11 can be removed. The treatedproducts drop from the hood into a product collection chamber 32, andthence .across a travelling grate 33 to a screw conveyor 34 via which itis taken to storage.

When the above-described kiln is used to manufacture cement, often asmall portion of the hot gases is taken from the dust collecting hoodand forced through the travelling grate to cool the clinker, andreturned through the rotating shell to the dust collecting hood. Thishot gas stream is referred to as secondary air, and usually represents avery small flow through the kiln, since substantial amounts of air mustbe used to provide the necessary oxygen for burning the fuel.

The above-described kiln is more or less typical of one that would beemployed for manufacturing cement. In such manufacture, a slurry isintroduced into the upper end of the rotating shell 11 via feed pipe 23and travels down the kiln as the shell rotates, While the hot gaseousproducts of combustion along with a small amount of secondary air, movecounter-current thereto up the shell toward the dust collecting hood 22.As the slurry progresses down the shell, water and carbon dioxide aredriven out of the slurry and the carbonate material is transformed intoa fused product which is discharged from the bottom of the firing hoodas clinker."

Normally, the hot products of combustion travelling up the shell 11enter the dust collecting hood 22 and from there pass through .abreaching 35 to a dust precipitator 36 wherein the solid particles areremoved. Subsequent to the passage through the precipitator, the hot gasstream passes through a second breaching 37 to induced draft fan 38 fromwhich it is expelled to the atmosphere through a stack (not shown).

In a conventional process the hot gases within the rotating shell, withthe exception of the secondary air, are produced by sucking air throughthe inlet of the primary blower fan 31, mixing it with fuel in fuelhopper 30 and discharging the combustible mixture into the lower end ofshell 11 where it is burned in the combustion Zone. Only a small portionof the hot gases passing through the shell, as mentioned above, can besecondary air, and usually this represents less than 10% of the gas massflow through the shell.

It is actually in this area of heat generation that the instantinvention departs radically from the conventional processes since no airis used to support the combustion of the fuel as is the case inconventional processes. Instead according to the practice of thisinvention, the hot gas products discharged by the induced draft fan 38are routed into plenum chamber 39 whose lower end 40 communicates with asubterranean concrete conduit 41 which returns to the lower end of thekiln. A large portion of the hot gases in the conduit are passed throughsecondary blower 42 and returned to the lower end of shell 11 viaconduit 43 for recirculation therethrough, usually passing through thetravelling grate structure 33 to cool the clinker slightly.

The remainder of the hot gases in conduit 41 is removed through conduit44 which connects to the inlet'of the primary blower fan 31, which in .aconventional system has a fresh air inlet. These hot gases entering theblower fan are principally composed of CO and H 0 (vapor), but some CO,ash carbon and uncombusted fuel products are also present. Since thesehot gases contain no oxygen they will not support the combustion offuel. In the instant invention, the lack of oxygen in this hot gaseousmixture is overcome by adding controlled amounts of oxygen to the hotgases in conduit 44 via an oxygen supply line 45 and valve 46 prior tothe entry of these gases into the inlet of the primary blower fan. Avalve 47 in line 44 is used to control the volume of hot gases passingthrough conduit 44 so that a proper ratio of hot gases .and oxygen canbe obtained through automatic operation of these valves with controller48.

Since the hot gases passing through the supply conduit 44 contain onlyminor amounts of CO and uncombusted fuel products plus a small volume ofcarbon particles, the addition of oxygen to this gas stream is nothazardous, .and can be accomplished safely. Further, since the oxygen isblended with the hot gases prior to the combination of fuel therewith,the oxygen mixture in the hot gases may vary from 1% to 20% prior tocontacting the fuel. This amount of oxygen is in a proportion similar tothe amount of oxygen in air, and represents no greater hazard thanburning the fuel with hot air at the same temperature. Of course, thistechnique is one of the principal ways the instant invention gainsoutstanding efficiency-combining the hot synthetic mixture composed ofoxygen and hot combustion products with the proper amount of fuel forheat generation by subsequent combustion.

In operation, the synthetic gas mixture entering the inlet of theprimary blower fan 31 is discharged into the burner pipe where it ismixed with fuel from hopper 30, and discharged into the combustion zonewithin shell 11 where it is burned. It may be desirable to mix the fuelwith the synthetic gas mixture near the inboard end of the burner pipeso that the burner pipe will not become overheated.

Since the majority of the synthetic gas mixture is composed of hotcombustion products, its temperature is substantially higher than theair used in conventional processes, and it will substantially improvethe combustion efiiciency of the novel process. Conventional processesoften try to recoup some efficiency by warming the inlet air with thehot combustion products in heat exchangers, but such a technique ofiersa very limited improvement. Further, a substantial fuel saving iseffected by this novel process since it is not necessary to raise thetemperature of a high volume of inert nitrogen to combustiontemperature, and thereafter suffer .a heat loss when releasing this hotgas to the atmosphere via the stack to make room for more nitrogen.Because of this advantage, much less heat need be added to the kilnsystem employing this invention than in a conventional process, andfurther the gas velocities may be substantially reduced for moreefiicient heat transfer, thereby improving the efficiency still further.

Ordinarily, this novel system contemplates a constant volume of gascirculating through the kiln and, since additional gas volume isgenerated by the combustion process and the addition of oxygen, it isnecessary to remove the excess gas volume of the system to keep it at aconstant volume. This removal is accomplished through an exhaust fan 49,driven by a variable speed motor 50 with its inlet communicating withthe plenum chamber 39. The speed of the exhaust fan will normally becontrolled by static pressure transducers within the plenum chamber,which, through a control unit, will vary the motor speed to keep aconstant pressure within the system, thus insuring a constant volume.

The outlet 51 of the exhaust fan 49 leads directly to a gas recoverysystem composed of two gas absorbing columns. Both columns, which aredesigned for countercurrent flows, are filled with a suitable packingmaterial or contain bubble plates to effect intimate contact between thehot exhaust gases and the absorbing liquids. In the first column whichthe hot gases from the exhaust fan enter, is the water condensing column52, which uses a coolant such as a cool water to treat the hot exhaustgases. As the gases are cooled by the cool absorbant flowin-gcounter-currently through this column, the water vapor in the gases,which comprises about 50% of the exhaust gases, is condensed out leavingonly a gaseous stream of carbon dioxide emanating from the top of thecolumn. A small amount of the carbon dioxide may also be absorbed in thewater as well as minor amounts of acidic oxides, ash, and other foreignmaterials from the combustion process. A recycling system 53 providesfor the recirculation of the condensing water to improve the efliciency.

The carbon dioxide remainder of the exhaust gases leaves the top of thecolumn 52 via line 54 and enters the bottom of the CO absorbing column55 which uses an absorbing liquid, such as ammonium hydroxide, to absorbthe CO stream entering this column. A small amount of unabsorbed gasesemanates from the top of the column through vent pipe 56, but is a verysmall amount of pure inert gaseous materials. Also, a recycle system 57for the hydroxide absorbing liquid is incorporated on this column. Theproduct laden streams are removed from the bottom of the respectivecolumns through lines 58 and 59.

Actually, the collection of carbon dioxide and water from the exhaustgases is made practical by the high concentration levels of thesematerials in the exhaust gas. Normally the compositon of this gas will bcomposed principally of carbon dioxide and water vapor in an equalamount. In conventional processes, gases leaving the kiln Via the stackcontain only '6%-20% of these useful products since they are composedprincipally of nitrogen entering the system with the air used to supportthe combustion. Carton dioxide and water vapor represent nearly of thegas volume exhausted and the recovery of these valuable products willimprove the economics of practicing the invention. Further, being asubstantially closed system, little, if any atmospheric contaminationresults from using this novel process, which is becoming of increasingconcern in metropolitan areas, even when using low equality fuels, suchas those with high sulfur content.

The above description centers around the practice of the-invention in acement kiln, but it should be appreciated that this invention could beemployed in steam genated plants, other boiler systems and installationshaving continuous or cyclic combustion processes, with the sameadvantages.

The following example will help illustrate the principles of thisinvention in practice.

In a typical cement kiln, 1,510,000 pounds of clinker a day are producedfrom 13,735,335 pounds of raw material introduced into the kiln. Thiswould show a material efficiency of 11.4% for product to raw materialratio. The materials not included in the product appearing as wastematerials in the efiluent stack gases flowing through the systemsbetween a conventional kiln an done operated according to this inventiondemonstrates the following differences using the same quantities offuel, even though in actual practice the greater thermal efiiciencies ofthis invention would substantially reduce the amount of fuel consumedand proportionally reduce the gas volume.

Conventional process Invention process CO 1,315,000 H O 1,445,380Clinker 1,510,000

Total 4,270,3 80

By the same arithmetic used previously it is apparent that from thestandpoint of clinker total materials standpoint the material efiiencywould be 35.5% If th recovery of water and carbon diovide is allowed asproduct also, the material efficiency approaches 100%.

Some of the more sophisticated advantages of this invention are notreadily apparent and etfect an actual reduction of fuel requirementsmuch greater than would be anticipated. The slow regulated movement ofthe synthetic gaseous mass through the system having larger heatcapacity and resistance to radiant energy losses improves the thermaletficiency of the system.

The diathermous character of synthetic gas mixture, being primarilycarbon dioxide and water vapor with oxygen added is superior to that ofair since nitrogen, the principal component of air, is much lesselfective against impeding radiant energy losses. Likewise the specificheat capacity of the synthetic gas mixture is superior to that of air.

I claim:

1. An improved closed combustion process for rotary cement kilns whereinno air is employed to support combustion comprising:

(a) forming a synthetic gaseous mixture composed principally of carbondioxide and water vapor;

(b) continuously circulating said synthetic gaseous mixture without theaddition of air through the cylindrical shell of said cement kiln;

(c) adding substantially pure oxigen to said synthetic gaseous mixtureto form a combustible mixture;

((1) adding fuel to said combustible mixture under combusting conditionsto add thermal energy to said circulating gaseous mixture whereby directheat exchange with the cement in said kiln can be effected; and

(e) withdrawing only that portion of said circulating gaseous mixturenecessary to provide gas volume for additional synthetic gaseous mixtureformed by the combustion of fuel and oxygen and kiln operation wherebythe closed system will operate with a constant volume of syntheticgaseous mixture.

2. The process as defined in claim 1, wherein the synthetic gaseousmixture is principally carbon dioxide and water vapor in similaramounts.

3. The process as defined in claim 1 wherein the quantities of oxygenadded to the circulating synthetic gaseous mixture is of 1 to 50% of thevolume thereof and in excess of that needed to combust the fuel.

4. The combustion process as defined in claim 1 wherein the portion ofsynthetic gaseous mixture removed therefrom is treated to recover the COand H 0 contained therein.

References Cited UNITED STATES PATENTS 797,506 8/1905 Eldred. 1,916,9807/1933 Horvitz. 1,931,817 10/1933 Hogan et al. 2,016,815 10/1935Gilmore. 2,111,783 3/1938 Hults 26353 XR 2,210,482 8/1940 Derrom 263532,740,693 4/1956 Pomykala. 2,865,344 12/1958 Firl. 2,980,082 4/1961Firl. 3,232,592 2/1966 Lohman. 3,074,707 1/ 1963 Humphries et a1.

FREDERICK L. MATTESON, ]R., Primary Examiner H. B. RAMEY, AssistantExaminer

